A touch screen is a device that can detect an object in contact with or in proximity to a display area. The display area can be covered with a touch-sensitive matrix that can detect a user's touch by way of a finger or stylus, for example. Touch screens are used in various applications such as mobile phones and other mobile devices. A touch screen may enable various types of user input, such as touch selection of items on the screen or alphanumeric input via a displayed virtual keypad. Touch screens can measure various parameters of the user's touch, such as the location, duration, etc.
One type of touch screen is a capacitive touch screen. A capacitive touch screen may include a matrix of conductive lines and columns overlaid on the display area. In mutual capacitance sensors, the capacitance between each line and column of the matrix may be sensed. A change in capacitance between a line and a column may indicate that an object, such as a finger, is touching the screen or is in proximity to the screen near the region of intersection of the line and column.
Mutual capacitance sensing circuits employ a “forcing” signal applied to a column conductor of the capacitive touch matrix and sensing of the coupled signal on respective line conductors. Since the capacitance change caused by a finger is small, on the order of 50˜100 fF (typically 5% of Cs=1˜2 pF), noise reduction is important in achieving satisfactory operation. There are two main sources of noise, including intrinsic noise generated by the electronics and external noise injected from the environment. In order to achieve a high signal-to-noise ratio (SNR), both sources of noise must be taken into account.
One known architecture uses a capacitance-to-voltage converter followed by a unity gain low pass filter and an analog-to-digital converter. During each cycle of operation, a forcing signal is applied to the capacitor to be sensed, and charge stored in the capacitor is converted to voltage by the capacitance-to-voltage converter. The low pass filter provides noise filtering, and two analog-to-digital conversions are performed on each cycle to provide information about the noise and the signal plus noise. It can be shown that the resolution of the system is limited by the magnitude of the signal at the output of the low pass filter in relation to a quantization error of the analog-to-digital converter. With this arrangement, there is a conflict at the output of the low pass filter between the magnitude of the signal, which is minimized to provide headroom for external noise, and the level of white noise which should be greater than 1.5 LSB of the analog-to-digital converter. Accordingly, there is a need for improved capacitance sensing circuitry.